The photorefractive effect is the process by which light alters the refractive index of a material. Photorefractive effect and photorefractive materials are discussed by D. Pepper et al in Scientific American, page 62 (October 1990), and by P. Gunter and J.-P. Huignard, Photorefractive Materials and their Applications, Volumes 1 and 2, Springer-Verlag, 1988. Photorefractive materials have wide ranging applications including hologram cameras, optical image processing, spatial light modulators, optical interconnects, phase conjugate mirrors, and high density data storage.
The photorefractive effect is related to electrooptic phenomena. An introduction to electrooptics and other nonlinear optical phenomena, as well as to nonlinear optical materials is provided by Nonlinear Optical Properties of Organic Molecules and Crystals, Volumes 1 & 2, edited by D. S. Chemla and J. Zyss, Academic Press, 1987. The relationship between photorefractive effect and nonlinear optics is described in the Scientific American publication referred to above, and by J. Feinberg in Physics Today, page 46 (October 1988). Briefly, in a photorefractive material, light, usually from a laser source, causes a migration of electrons from one region to another, creating an inhomogeneous charge distribution, and hence an electrical field. This alters, or modulates, the refractive index of the material by way of the electrooptic effect. Thus, a photorefractive material exhibits electrooptic effect, as well as electronic charge transport phenomena generally termed photoconductivity.
Traditionally, photorefractive materials have been inorganic crystals such as, for example, lithium niobate, barium titanate, strontium barium niobate, CdS, GaAs, and the like. However, these materials are associated with problems such as high costs, limited efficiency in growing suitable crystals, and restricted shapes and sizes without much possibility for further processability of the crystals. Recently there has been a growing interest in the preparation of organic photorefractive materials. Organic materials generally may not have the above-mentioned problems. Furthermore, some electrooptic figures of merit for organics, such as, for example, n.sup.3 r/.epsilon., are higher for organics than for inorganics. Additionally, the potential for controlling the spatial sensitivity in organic materials is much higher than in inorganics. This is so because a wide range of organic sensitizers for photocharge generation are available in the visible and near infrared regions, and therefore the wavelength at which photoconduction occurs can be chosen as desired.
K. Sutter et al, Journal of Optical Society of America, B, Volume 7, page 2274 (December 1990) describe the photorefractive properties of the single crystals of an organic material, 2-cyclooctylamino-5-nitropyridine doped with 7,7,8,8,-tetracyanoquinodimethane (TCNQ). S. Ducharme et al, Physical Review Letters, Volume 66, 1846 (1991) describe the observation of photorefractive effect in an electrooptic polymer, bisphenol A-diglycidyl ether/4-nitro-1,2-phenylenediamine, when doped with an organic photoconductor, diethylaminobenzaldehyde diphenylhydrazone. J. S. Schildkraut, Applied Physics Letters, Volume 58, page 340 (1991) discloses a three component organic photorefractive material, one of which is an electrooptic polymer containing a nonlinear optical chromophore, 4'-dialkylamino-4-methylsulfonyl stilbene (Formula 1). This is mixed with the sensitizer, N,N'-bis(2,5-di-tert-butylphenyl)-3,4,9,10-perylenedicarboxamide (Formula 2), and the photoconductor, 1,1-bis(4-di-p-tolylaminophenyl)cyclohexane (Formula 3). ##STR2##
While the above organic systems exhibit the photorefractive effect, they are mixtures or guest-host materials, and as such are amenable to problems associated with guest-host systems. For example, guest-host systems generally suffer from loading problems. In other words, the amount of component materials that can be mixed together and be retained as a homogeneous system without phase separation may be limited. In addition, while the different components may be good performers individually, compatibility among them may not exist or may be difficult to achieve. Furthermore, electrooptic effect in a material depends on the efficiency of dipole orientation, and the presence and amount of the other components, particularly large amounts of the photoconducting material, may adversely influence the dipole orientation of the electrooptic material. Some electrooptic materials are poled in an electrical field, and the photoconductor may influence the efficiency and results of the poling. Thus, there is an interest in the development of novel organic photorefractive polymeric materials that are not made up of physical mixtures and guest-host systems.
U.S. Pat. No. 4,999,809 discloses a photorefractive layer comprising a homogeneous organic photoconductor containing electrooptical moieties. The electrooptical moieties are covalently bound to a polymer backbone. The photorefractive layer disclosed by Example 1 in the same patent comprises the polymer of Formula 4, a sensitizer, and a charge transporting agent. ##STR3##
However, there is a growing interest to have the photoconductive and electrooptic parts, covalently linked as discrete units to the same polymer backbone. This may not only result in a homogeneous system but may also avoid the need to add an external transporting agent. It is well known in the art to have polymers with different functionalities as discrete units on the same polymer chain. However, due to issues such as, for example, the compatibility considerations discussed above, it is not known to have discrete photoconductive and electrooptic components in the same polymer.